Life Down to the Last 1 x 10-18 of a Second

We wear watches, communicate through cell phones and computers, and use GPS while driving, walking, and flying. We need and expect accurate timing and synchronization so that phone calls and text messages aren't delayed when, for example, an entire geographic region communicates a natural disaster. Atomic clocks achieve this type of precision. In 2010, the world’s most accurate clock debuted and demonstrated that, among other things, time does move faster in weaker gravitational fields.1, 2, 3 

Cesium atomic clocks are the official timekeepers in use worldwide (see excellent review by Lee).4 However, cesium (CAS Registry Number® (RN): 7440-46-2) did not come to define the second until 1967, when the 13th General Conference on Weights and Measures recognized the superior performance of atomic frequency standards. Atoms have natural resonances in which electrons hop (oscillate) between two states by emitting or absorbing electromagnetic waves of a set frequency.2 The cesium atomic clock relies on a microwave frequency of 9.2 billion oscillations per second. This frequency defines the second.

Atomic fountain clocks have emerged in recent years with laser cooling technologies that slow the movement of the atoms, achieving an accuracy of two parts in 1015. The most accurate atomic clock to date utilizes aluminum (CAS RN: 7429-90-5) and marks time to within one second every 3.7 billion years.3 This so-called quantum logic atomic clock uses ultraviolet lasers instead of microwaves.

Physicists were able to demonstrate the effect of the Earth’s gravitational field on time by hoisting one superaccurate clock several centimeters above another. Time passed more slowly for the higher clock, thus detecting a difference in gravity. Until previously, it took measurements near the speed of light or in exceptionally strong gravitational fields such as black holes to decipher a difference in time. Beyond detecting that our hair ages faster than our toenails, superaccurate clocks can detect oil beneath the Earth’s surface.5 Instead of extensive drilling, the reduced gravity of oil can be detected by comparing differences between superaccurate clocks.

Use SciFinder® and STN® to explore the many types and uses of atomic clocks to be found in the CAS databases.

Contributed by
Kathryn J. Meloche, Ph.D.
CAS Public Relations


  1. The Top 10 ScienceNOWs of 2010. Science 2010, 330, 1603. DOI: 10.1126/science.330.6011.1603. SciFinder link.
  2. Cho, A. Superaccurate Clocks Confirm Your Hair Is Aging Faster Than Your Toenails. ScienceNOW. (accessed January 10, 2011).
  3. Chou, C. W.; Hume D. B.; Koelemeij, J. C. J.; Wineland, D. J.; Rosenband, T. Frequency Comparison of Two High-Accuracy Al+ Optical Clocks. Phys. Rev. Lett. 2010, 104, 070802. DOI: 10.1103/PhysRevLett.104.070802. SciFinder link.
  4.  Lee, H. S.; Kwon, T. Y.; Park, S. E.; Choi, S; Park, Y. Research on Cesium Atomic Clocks at the Korea Research Institute of Standards and Science. J. Korean Phys. Soc. 2004, 45, 256-272. SciFinder link.
  5. World’s Most Precise Clocks Test Relativity. September 24, 2010. (accessed on January 10, 2011).

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